Tiller dynamics and self-thinning in grassland habitats

The tiller dynamics and weight-density relationships of grasses were investigated in sown monocultures and in an established grassland both in full light and under shade. Monocultures of Festuca pratensis and Lolium perenne sown at high densities conformed to the-3/2 Power Law during establishment, but once the maximum standing had been reached the trajectory of the thinning line switched to a slope of approximately-1 when weight was plotted against density on logarithmic scales. Subsequently the populations showed a cyclical pattern of tiller weight and density bound by a line related to the maximum standing crop but also close to the-3/2 thinning line.The natural populations also showed a cyclical pattern of tiller weight and density at combinations very similar to those in the sown monocultures and close to a thinning line with an intercept (log c) of 4.4-4.7 assuming the generality of the -3/2 Power Law as w=cN ^-3/2. Tiller dynamics were very similar on all the plots despite differences in the species composition of each plot. The greatest seasonal variation in tiller number occurred under the shade of birch trees, perhaps due to resource depletion caused by the tree canopy during late summer and the presence of tree litter during the winter. The artificial shade treatment had little effect on plant yield but caused a significant change in the composition of the vegetation. It is suggested that mixed populations of grasses behave in essentially the same fashion as monocultures and that vegetational change might be analysed as a multispecies participation in a mutual cyclical proces of tiller death and regeneration.